In order to improve the tolerance and the comfort of ophthalmic lenses, in particular contact lenses, considerable effort is undertaken. Aspects of interest in this respect are the wettability of an ophthalmic lens with lacrimal fluid, the resistance to proteinaceous deposits, biocompatibility, etc. Generally, for the manufacture of ophthalmic lenses specific lens materials are used which are highly hygroscopic and have a high wettability. Ophthalmic lenses also may be plasma treated, e.g. by exposing them to an oxygen plasma, in order to modify the lens surfaces and to improve the wettability thereof and to increase the resistance of the surface against contaminations. In another approach ophthalmic lenses may be provided with a coating in order to obtain the desired lens properties.
Providing an ophthalmic lens substrate, in particular a contact lens substrate, of one material with a coating of a different material has been proposed as a strategy for making contact lenses for a variety of reasons. The coating may offer properties that are different from those of the lens substrate and which are particularly desirable at a lens surface apart from the properties of the lens substrate material itself. Properties such as wettability, resistance to proteinaceous deposits, biocompatibility, UV shielding and other desirable lens characteristics and properties may conceivably be addressed by a specific coating. However, there may appear problems associated with coating lens substrates which include repeatability of both polymer type and coating thickness. Therefore, coated ophthalmic lenses, in particular contact lenses, are routinely inspected for the quality of the coating. Such inspection is also carried out e.g. when new coating materials are tested or in order to determine which coating thickness is necessary to achieve the desired properties, in particular with regard to wettability, resistance to proteinaceous deposits, aging, mechanical endurance etc. For example rubbing tests are performed manually, the results of which are very subjective and inaccurate. Such tribological methods applied by an experienced test person can only provide some qualitative results, which may somewhat be indirectly representative of the material properties of the coating and which may be highly subjective.
The wettability or hydrophilic properties of an ophthalmic lens with or without coating may e.g. be determined by measuring the contact angle in between the inspected surface of the lens substrate and a wetting liquid, usually water or a lacrimal fluid. A small measured contact angle indicates a good wettability and a good resistance to proteinaceous deposits. The measurement of the contact angle, however, is a rather cumbersome and inaccurate method for a determination of the quality of a surface of an ophthalmic lens. In particular with coated lenses the method is rather insensitive to changes of the coating, such as e.g. coating time, coating thickness, hydrophilicity, etc. Therefore, there exists a strong desire for a more convenient measurement method, which provides more accuracy and has a broader field of application with regard to the inspection and classification of the quality of a surface of an ophthalmic lens, which may be provided with a coating.